1. Field of the Invention
This invention relates in its broadest aspects to the apparatus and processes for the removal of dissolved salts from liquids by electrodeionization (EDI). In a narrower aspect, the invention relates to an improved electrodeionization process, to wit: the electrodeionization reversal process where the periodic reversal of the electrical polarity is employed.
2. Description of the Prior Art
The electrodeionization apparatus consists of an electric membrane demineralizer with its stack of alternating ion-selective membranes and ion-depleting and ion-enriching compartments in which at least the ion-depleting compartments are filled with ion exchange beads. The EDI process is useful in general to treat waters or gases containing ionizable contaminants, for example to produce ultrapure water, or to remove carbon dioxide from air. The use of electrodeionization (EDI) is disclosed in U.S. Pat. Nos. 2,815,320 (Kollsman), 3,149,061 (Parsi), 3,291,713 (Parsi), 3,330,750 (McRae et.al.), and many others.
EDI for the deionization of water requires extensive pretreatment of the feed water to remove ions which tend to cause precipitates to form in the chambers of the EDI stack, and to remove organic and other contaminants which would tend to collect on the membranes and on the resin, and cause the performance of the EDI stack to suffer. In particular the feed water to an EDI stack requires softening and organic scavenging.
One type of scaling is caused by precipitation of certain sparsely soluble salts, for example calcium carbonate or calcium sulfate, when their solubility is exceeded in the EDI apparatus. Calcium carbonate may form a scale at the cathode, where the electrolysis of water results in the generation of base and hydrogen gas. The base can react with bicarbonate to form carbonate, which in turn can form insoluble calcium carbonate or magnesium basic carbonate. Most natural waters contain calcium and magnesium ions. Other basic precipitates may also form. A similar effect can result from "splitting" of water at the ion-exchange membrane, ion exchange bead, and water interfaces, a phenomenon commonly known as polarization, which results in pH changes that can lead to precipitation within the stack. All of these forms of precipitates are commonly referred to as scale. Build-up of scale can result in an increase in the resistance to the flows of electricity and water through the stack. Also, physical damage can be inflicted on stack components by severe scaling.
Deposition of colloids, organic contaminants, and othher impurities on the surface of the membranes and ion-exchange beads generally results in an increase in electrical resistance; this may also result in an increase in the hydraulic resistance in the compartments of the stack and in a decrease in current efficiency. Some of these organic materials may also be absorbed in the ion-exchange membranes and/or beads as well, leading to a decrease in the overall performance of the stack.